A plurality of different systems concepts are known for solid state lasers and solid state fiber amplifiers. The book by Walter Koechner, Solid State Laser Engineering, Springer Verlag (Publishers), 5th edition, 1999, gives a good overview. Originally, the active element of a solid state laser had a cylindrical rod shape, and the optical excitation energy was generated using strong gas discharge lamps. However, semiconductor lasers are predominantly being used currently for the optical excitation of solid state lasers. In using excitation by semiconductor lasers, a substantially higher efficiency may be achieved in the conversion of electrical input power into laser power. Even in the excitation of solid state lasers using gas discharge lamps, other geometric shapes were known besides the cylindrical rod shapes of the laser-active solid, such as the round slab laser or the rectangular slab laser. However, excitation using semiconductor lasers has led to a whole series of new geometric shapes of the solid state laser-active materials used, and to new systems concepts, such as the disk laser (discussed, for example, in European Patent 0 632 551), the microchip laser, the non-planar monolithic ring laser or the “grazing incidence slab laser” (discussed, for example, in U.S. Pat. No. 6,002,695 and U.S. Pat. No. 5,315,612).
One common aim of almost all systems concepts for solid state lasers or solid state laser amplifiers is to achieve highest possible beam quality of the generated or amplified laser radiation. The beam quality of a laser, which may be equated to its focusability, is a decisive quality criterion concerning lasers. However, in addition to the aim of achieving the highest possible beam quality, other aims are striven for, such as high efficiency and a simple and compact design.
However, all the known concepts have certain disadvantages. Even in the case of excitation using semiconductor lasers, the traditional rod laser has a thermal lens and photoelastic birefringence, which leads to a bad beam quality at higher laser power.
on the other hand, if the design is appropriate, a slab laser having a zigzag optical path has only one very slight thermal lens and hardly any stress birefringence. However, for this it is presupposed that the crystal of the slab laser has a large aspect ratio. The aspect ratio is the quotient of the great crystal dimension transverse to the laser beam (“height”) and the small crystal dimension transverse to the laser beam (“thickness”). A large aspect ratio of the crystal dimensions as a result has a great height of the laser crystal, because the crystal cannot be made as thin as desired, since otherwise its mechanical rigidity would suffer. However, a great crystal height leads to an oscillation buildup of high transverse modes, which leads to poor beam quality. In the past there were many experiments on the subject of achieving good beam quality by the use of special optical resonators (e.g. unstable optical resonators or folded optical resonators). However, in practice none of these methods was able to succeed, since all are very sensitive to the least bit of interference of the laser beam. A further disadvantage of the conventional slab lasers is that the slantwise passage of the laser beam through the crystal's end faces gives rise to astigmatism. Finally, all the known slab lasers have aberrations which are brought on by deformation of the crystal's surface. In the case of most slab lasers, the crystal is pumped homogeneously over the two totally reflecting surfaces, so that these surfaces are not able to be cooled any more by solid state contact cooling. The totally reflecting surfaces are therefore cooled using a liquid (usually water), which is transparent to the pumping light. However, in a laser system a liquid always represents a risk factor since, in the case of leakage, great damage may be created to the optics and the pumping light source. A further disadvantage is that the tips of the slab crystal are deformed by thermomechanical stress. If only one of the two totally reflecting surfaces of the slab crystal is pumped, the other crystal surface may be used for contact cooling of the solid. Though, when it comes to this design, a variation of the pumping power leads to a deflection of the laser beam. The reason for this is the asymmetrical, changing deformation of the crystal surfaces at which the laser beam is totally reflected. This effect, known as “beam steering”, in the amplifier operation leads to an undesired change of direction of the laser beam, and in the laser oscillator operation it leads to the optical resonator's going out of alignment and to the decline of the output power. In the history of solid state lasers, a great many firms have occupied themselves with the development of slab lasers, but almost all of them have given up the corresponding purposes, since the problems turned out to be insolvable.
The disk laser combines some of the advantages of the rod laser and the slab laser. The transverse dimensions of the crystal are small, similar to the rod laser. Since the beam runs perpendicular to the isotherms in the crystal, practically no thermal lens appears, just as in the slab laser. The beam quality is good if the pumping light is focused on a small, round spot in the disk, since then there is no buildup of higher transverse modes. In order to achieve a sufficiently high power using the small pumped area, the disk must be made very thin, because only in that manner can the thermomechanical stresses be held low. However, a thin disk absorbs only a part of the pumping light, so that the pumping light has to traverse the disk several times if there is to be a sufficient absorption overall. The strong focusing of the pumping light on one round spot requires great expenditure, because semiconductor lasers themselves have a rectangular aperture having an extremely large aspect ratio. Therefore, anamorphotic optics are required to generate a round beam. The multiple traversals of the pumping light through the crystal require additional optical systems which increase the expenditure once again. These two optical systems lead to a large overall type of construction of the laser, as well as resulting in high costs and limited reliability.
Even the monolithic ring laser requires a round pumping light beam, and it thus has the same disadvantages as the disk laser with respect to pumping light formation. In one known system, at any rate, as in the case of the disk laser, the thermal lens is avoided (discussed, for example, in published German Patent Application No. 197 22 943.3).
The grazing incidence slab laser cannot, in the actual sense of the term, be denoted as a slab laser, since it has a considerable thermal lens. To be sure, as in the usual slab laser, the thermal lens is compensated for in the direction perpendicular to the reflecting surface; however, in the direction orthogonal to that, a strong thermal lens effect appears. The reason is that, in this direction, a constant temperature is not present in the laser crystal. The light of a semiconductor laser ingot is focused into the laser crystal using a cylindrical lens, or is conducted into the crystal by direct coupling. In this context, the pumping light enters the crystal through that particular crystal surface at which the laser beam of the solid state laser is also reflected. Since this crystal surface is needed for the coupling in of the pumping light, it is not able to be simultaneously used as the cooling surface for contact cooling. Therefore, the crystal is cooled at the two large surfaces perpendicular to this surface. However, this leads to a temperature gradient transverse to the laser beam, and thus to a thermal cylindrical lens.
It may be said in general that, at this time, there are two kinds of solid state lasers pumped by semiconductor lasers. On the one hand, lasers exist in which the geometry of the active medium is rectangular and is thus adapted to the geometry of the semiconductor laser ingot. These are the slab lasers, which, however, have strong aberrations and a bad beam quality. On the other hand, lasers exist in which the pumped volume of the active medium is adapted to the round shape of the TEM00 mode. These lasers (longitudinally pumped rod lasers, disk lasers, monolithic ring lasers), however, require a costly beam formation of the pumping light of the semiconductor laser, in order to transform the rectangular beam cross section of the light that leaves the semiconductor laser, into a round beam cross section of the cylindrical region of the active medium that may be pumped. These beam transforming optical systems are very costly and lead to power loss, as discussed, for example, in the journal Laser Focus World, (Laser Focus World, 98 Spit Brook Road, Nashua N.H. 03082, USA), December 2001 Edition, page 115 ff, Laser Focus World, May 1998 Edition, page 32 ff, as well as Laser Focus World, May 1994 Edition, page 46 ff.
The object of the present invention is to provide a solid state laser or a solid state laser amplifier which does not have the above-described disadvantages of the known systems. It should not have a thermal lens nor stress-induced birefringence, it should supply a diffraction-limited laser beam, but should nevertheless make do without anamorphotic beam formation for the pumping light of the semiconductor laser, i.e. the systems concept should be adapted both to the round TEM00 mode and to the rectangular beam profile of the semiconductor lasers. In addition, the laser should be constructed simply and cost-effectively, the laser-active solid and the other optical components should have no astigmatism and no “beam steering” for the laser beam, the laser should have a high efficiency and be cooled by solid state contact cooling.